This invention relates to thermally insulated tanks and containers suitable for the storage of liquefied gases, particularly liquid natural gas (LNG). The invention is particularly concerned with the provision of a cryogenic thermal insulation system of the above type which controls the amount of liquid penetration into the permeable insulation and provides a venting system for gases generated from such penetrating liquid, and avoiding degradation or rupture of the insulation material.
The use of an internal tank wall insulation system for cryogenic liquid storage vessels such as for liquefied natural gas is highly desirable since such a system permits the use of lower cost non-cryogenic tank structural materials, reduces cooldown losses during filling and is easy to maintain. However, the thermodynamic and hydrostatic conditions of stored cryogens permits liquid penetration into permeable insulations. Although the accompanying thermal degradation would be tolerable for some applications, if the penetrating liquid were stable and did not flow, in most instances however, the insulation becomes sufficiently permeable, because of pressure and temperature cycling, that gravity induced circulation is established in each sealed insulation block. Much deeper liquid penetration then occurs, resulting in intolerable thermal degradation, and can even cause the tank wall temperature to drop sufficiently to present structural problems with non-cryogenic structural wall material.
In addition, the conversion of cryogenic liquid such as LNG penetrating the permeable insulation, into large volumes of gas as the liquid warms up and converts to the gaseous state, results in expansion sufficient to tear or rip out pieces of the insulation material during warm-up of the system.
The prior art has employed various concets for thermal insulation of cryogenic containers in an effort to solve the above noted problems resulting from the penetration of cryogenic liquids from the container into the surrounding insulation. Thus, for example, in U.S. Pat. No. 3,411,656 there is disclosed a thermally insulated container for a liquefied gas having an outer wall of substantially fluid impermeable heat insulating material, an intermediate layer of completely fluid permeable material such as open cell flexible polyurethane foam which is provided with a vent to allow escape of gases from the layer, and an inner liner of a material of controlled liquid permeability such as a glass reinforced epoxy resin. However, such system has the disadvantages of requiring external venting directly from the intermediate insulation layer via a complex manifold system, and also requires the use of different types of insulating material including both the fluid impermeable and fluid permeable types.
Cryogenic liquid permeation processes as noted above and illustrated by the above patent can be theoretically eliminated by using a completely impermeable liner between the stored liquid and the insulation. Welded stainless steel liners have been proposed for this purpose. However, such approaches are costly and are of questionable reliability for large, longlife cryogenic systems. Thus, if even a slight liquid leakage occurs with this concept, severe damage to the liner and insulation can result from pressure buildup due to gas expansion within the insulation from evaporation and expansion during warming.
It is an object of the present invention to provide an efficient thermal insulation system for cryogenic containers and tanks. A particular object of the invention is the provision of a relatively simple thermal insulation system of the above type incorporating means which limits the region of flow of liquid cryogen into the thermal insulation layer and wherein a film of vapor is generated at a controlled depth of penetration of the cryogenic liquid, thereby stabilizing the degree of penetration of the cryogenic fluid into the insulation material, and providing optimum thermal performance.